In order to get the benefit for your mortgage interest deduction you need to be itemizing your deductions. So it's possible that at $11k of mortgage interest the total of your itemized deductions is not greater than the standard deduction, but that at $20k the itemized deductions DO exceed the standard deduction. If you're filing single the standard deduction is $12k, if you are married filing jointly it's $24k.
Please visit this comprehensive article for more information about the standard and itemized deductions:
https://ttlc.intuit.com/replies/3302313
